This invention relates to a method for the manufacture of a cast iron product having a high resistance to pitting.
A cast iron product such as, for example, a tappet or valve lifter generally used in an automobile engine system, is generally required to have both a resistance to wear and a resistance to pitting because it is frequently subjected to repeated friction and repeated impact. The Japanese Patent Publication No. 43-17497, published July 24, 1968, discloses the production of a wear resistant tappet made of cast iron.
According to the above mentioned publication, a cast iron containing 2.7 to 3.8% of carbon (C), 1.7 to 3.0% of silicon (Si), 0.5 to 1.5% of manganese (Mn), up to 1.5% of nickel (Ni), 0.2 to 1.0% of chromium (Cr), up to 1.0% of molybdenum (Mo), 0.01 to 0.10% of boron (B) and iron (Fe) being the balance, wherein the ratio of addition of B and Cr is within the range of 1:1 to 1:10, is first melted in an electric furnace. The melt is then cast into a mold using a chiller for chilling that portion of the casting which eventually defines an abutment face of the valve lifter for the engagement with a cam lobe so that a chilled structure can be imparted to the abutment face of the valve lifter while the remaining portion thereof represents a structure of gray cast iron. Thereafter, in order for graphite to be precipitated at the eventually defined abutment face of the valve lifter, the casting is annealed for 10 to 120 minutes at a temperature within the range of 850.degree. to 950.degree. C. Subsequent to the annealing, the annealed casting is machined leaving a finishing allowance, followed by the heating for 5 to 20 minutes at a temperature within the range of 800.degree. to 900.degree. C. for the subsequent quenching into oil. Finally, the quenched casting is tempered at 100.degree. to 250.degree. C. for not more than 120 minutes to render the abutment face to exhibit a martensite matrix containing carbide and annealed graphite dispersed therein.
It appears that the self-lubricating property and the improved resistance to wear of the cast iron product manufactured by the prior art method discussed above have resulted from the presence of the graphite which has been precipitated by annealing at a high temperature the carbide (Fe.sub.3 C), that is, the cementite structure, formed by the use of the chiller. While it has been found, as hereinafter discussed in connection with the present invention, that an excellent resistance to wear can be obtained if the amount of the cementite remaining in the base material is relatively great, the reduced amount of the cementite has been found resulting in the reduced resistance to pitting.
Moreover, according to the prior art method discussed above, the cast iron product subsequent to the machining step is subjected to the quenching treatment consisting of heating at 800.degree. to 900.degree. C. for 5 to 20 minutes and rapidly cooling in oil. Because of this quenching treatment, it has been found that not only is the matrix transformed to a martensite structure, but also a ferrite layer is formed as an intermediate layer. Since the ferrite layer is fragile and brittle, the presence of the ferrite layer constitutes a cause of "tapping wear" in which particles of the carbide drop from the base material and a cause of the reduced resistance to pitting. Therefore, the prior art method discussed above employs the addition of boron for the purpose of minimizing the formation of the ferrite layer.
Apart from the above discussed prior art method, U.S. Pat. No. 4,124,413, patented Nov. 7, 1978, discloses a method of producing a wear and pitting resistant cast iron, which method comprises casting a melt consisting essentially of 2.8 to 3.3% of C, 1.5 to 2.1% of Si, 1.0 to 1.5% of Cr, 0.6 to 0.8% of Mo, 0.2 to 0.55% of Ni, up to 0.05% of P and the balance being substantially all iron, to form a casting having a free cementite content of 20 to 40% by volume, and then hardening the resultant casting by heating it at 860.degree. to 950.degree. for 1 to 10 hours and then quenching it to produce a product having a Vickers hardness of Hv 750 to 900.
The quenching or hardening treatment employed in the last mentioned prior art method is also susceptible to the formation of the ferrite layer, and nowhere in the patent specification is disclosed an idea of and countermeasures for suppressing the formation of the ferrite layer.